What guidelines are you going by here? All the guidelines I'm aware of place M and R at the same level, as well as AO and NC-17 (Basic Instinct originally received an NC-17 rating, by the way).
Also check out this quote from Wikipedia
The AO (Adults Only) rating is seldom awarded; in the fifteen year history of the organization and thousands of products rated, only twenty-one titles have officially earned and kept the rating.
So are you really saying that only 21 of the thousands of games rated by the ESRB in the span of 15 years have had content similar to that of an R rated movie?
sort of: i'm saying that some (but not all) of those 21 games' content, if they were in movies, would probably get R rather than NC-17
also it's not a particularly good defense of how these ratings work to say that only 21 games have gotten AO ratings -- if anything that shows just how bad the self-censorship is in the industry (in terms of how much variety of content it prevents from being made). compare with the number of movies that have X / NC-17 ratings